Reference of Replacement Conditions and Pressure Drop Values for Drying Filters

2025-04-11

The drying filter is one of the most commonly used accessories in the refrigerant pipeline. It is mainly used to clean the harmful substances in the system, thereby protecting important components such as the expansion valve and the compressor. For the drying filter, the ability to adsorb moisture from the refrigeration system is a very important function.

In the refrigerant system, many harmful substances will enter the pipeline through various channels. The common ones are as follows:

During production or maintenance: At this time, the refrigerant system is open to the outside world. No matter how long the time is, water vapor and dust will enter the system pipeline. If there is no nitrogen protection during the welding process, a copper-oxygen film may even be generated, and there may be welding slag;
Refrigerant or refrigeration oil charging: If these substances themselves contain a lot of solid impurities and moisture, they will all be brought into the system and corrode the metal parts in the system;
During the operation of the refrigeration system: During operation, when the refrigerant contains moisture, it will hydrolyze to generate acidic substances, which will corrode the metal. The mixture of refrigerant and lubricating oil can dissolve copper and carry it to the compressor, forming a copper film on its metal surface, thus affecting the operation of the compressor.
Refrigeration oil: When the temperature is low enough, wax will precipitate from the refrigeration oil. Moreover, for the POE oil used in environmentally friendly refrigerants, it has strong hygroscopicity, so it is more likely to hydrolyze acidic substances.

The presence of the above substances will seriously threaten or damage the moving parts in the system, the cylinders that need to compress gas, and the performance of the refrigeration system, so they should be removed in a timely manner.

The main materials that play a role in the drying filter are:

Metal filter screen: Filters larger solid particles (mainly used in small and medium-sized systems such as R22 and those using mineral oil that are not sensitive to moisture);
Polyester filter screen: Filters tiny particles;
Glass wool: Filters tiny particles;
Molecular sieve: Filters larger particle solid impurities, mainly absorbs moisture, which is 3-4 times that of other substances, and can also absorb nearby acidic substances;
Activated alumina: Absorbs moisture, mainly acidic substances.
Activated carbon: Absorbs paraffin. However, since paraffin will only precipitate from the refrigeration oil when the temperature is lower than -35 degrees, it is not needed in systems operating at medium and high temperatures.

The characteristics of commonly used desiccants are as follows, and they are made into two forms according to their usage characteristics:

Small ball combination type: Many small balls made of desiccants are filled in a container inside the drying filter.
Rigid filter element type: The desiccant is made into a rigid filter element through a series of processes and then installed on the fixed support of the drying filter.

Performance and classification of the drying filter:
Filtering ability:
The filter screen is generally made of stainless steel, phosphor bronze or brass. For filtering gaseous refrigerant, 70-100 mesh is generally used, and for filtering liquid refrigerant, 60 mesh can be used. However, the filter screen before the expansion valve and the solenoid valve needs to use a fine filter screen of 120-200 mesh.
Moisture absorption ability:
The water content in the refrigeration system is the smaller, the better. And different systems are in different states before being closed. The length of the vacuuming time and the quality of the refrigerant charging will all have an impact.
Functionality:
The drying filter for the liquid pipe and the drying filter for the suction pipe (also known as the drying filter for compressor burnout) can both remove substances that may damage the system, but the focuses are different.
The drying filter for the liquid pipe mainly protects valve parts such as the stop valve and the expansion valve, and avoids the blockage of throttling elements such as the capillary tube. It is a standard configuration in the refrigeration system. Because the flow rate here is low, the filtering effect is good and the pressure drop is small, and there are many protected valve parts following it. Generally, a liquid sight glass follows behind the liquid pipe drying filter. There is also a kind of drying filter specially used to enhance the ability to clean the contaminated pipeline after the compressor burns out. It has a greater moisture absorption and acid removal ability than the normal one.
The drying filter for the suction pipe mainly protects the compressor. The drying filter for the suction pipe is not widely used in the industry yet. There are generally two situations where it is officially used as a system configuration:

On-site installation system:
Because on-site installation is different from factory installation, and the on-site environment is complex and the cleanliness is poor, it is best to install a drying filter on the suction pipe. (However, currently in China, for on-site installation products such as small and medium-sized split pipeline units, no drying filters are installed, and the quality is guaranteed by the construction unit's own process. Of course, this is for R22 products. I don't know if there is enhanced protection in this regard for environmentally friendly refrigerants.)
Large system:
Because the cost of the compressor is relatively high, it is worthwhile to install a removable drying filter on the suction pipe to protect the compressor.
In order to ensure that the pressure drop of the drying filter for the suction pipe is as small as possible (the pressure drop of the suction pipe has a relatively large impact on the system performance), it is generally larger than that for the liquid pipe (mainly referring to the flow capacity). Because the drying filter for the suction pipe is mainly used when the compressor burns out, it can adsorb a large amount of moisture, acidic substances or solid impurities in a short time. These substances will reduce the flow capacity of the drying filter, thus affecting its performance. Therefore, there are generally an interface near the inlet and outlet of the drying filter for the suction pipe to test the pressure loss of the drying filter. If the measured pressure difference between the inlet and outlet exceeds the following values, the drying filter should be replaced.
A liquid sight glass with a moisture indicator is usually installed after the drying filter. At this time, its indication meanings are as follows:
Green: There is no dangerous moisture in the refrigerant;
Yellow: The moisture content in the refrigerant before the expansion valve is too high.
Bubbles:

The pressure drop of the drying filter is too high;
There is no subcooling;
There is insufficient refrigerant in the entire system.
Consider replacing the suction filter when the pressure drop exceeds the following values:
Air conditioning system: 0.50bar;
Refrigeration system: 0.25bar;
Freezing system: 0.15bar.
During installation, the fluid direction must be consistent with the arrow direction on the filter label. The orientation of the drying filter is not limited, but the following points must be remembered: Vertical installation with the flow direction downward means that the refrigeration system can be quickly drained/emptied; for vertical installation with the flow direction upward, draining/emptying will take a longer time because the refrigerant needs to be drained out of the drying filter.

Replacement conditions of the drying filter:

The liquid sight glass indicates that the moisture content is too high (yellow).
The pressure drop of the filter is too high (bubbles appear in the liquid sight glass during normal operation).
The main parts of the refrigeration system have been replaced, such as the compressor.
Every time the refrigeration system is opened, for example, the flow port component in the expansion valve has been replaced. The old drying filter should not be reused. If it is used in a refrigeration system with a low moisture content or when it is heated, it will release moisture.
Replacement of the drying filter:
Close valve 1; Empty the filter; Close valve 4; Close valve 2.